Large workpieces, such as, door panels and countertops present particular problems in handling and especially in performing various finishing operations, such as for instance, sanding, filing, routing or the addition of hardware accessories. Vises have been employed in the past for holding smaller workpieces or articles so that a single person may perform different operations, but cannot be satisfactorily used in supporting larger workpieces. Accordingly, larger clamping devices have been proposed in an effort to support a workpiece in performing different finishing operations; however, such devices have not been found to be entirely satisfactory from the standpoint of rigidly positioning the workpiece in an upright position without danger of slipping or marring of the workpiece.
Representative of the approaches taken in the past is that disclosed in U.S. Pat. No. 4,138,099 to C. C. Englehart which provides a horizontal channel and a padded clamp centrally of the channel so that the panel can be positioned on one side edge in the channel and secured in place. One end of the channel is provided with wheels so that the door panel can be transported to different locations and the door raised into an upright position for hanging purposes but does not enable the door or other panel to be supported in an upright position in the performance of other operations. Other patents disclose apparatus similar to that of Englehart and, for example, reference is made to U.S. Pat. No. 4,050,671 to R. J. Coleman and U.S. Pat. No. 3,861,662 to C. W. Morse as well as U.S. Pat. No. 2,503,388 to C. L. Hedlund.
Another important consideration in the clamping of large panel structures is to provide a clamping device which will exert a substantially uniform pressure over a relatively wide area so as to avoid undue concentration and pressure at any one point. Nevertheless, the clamping device should be capable of compensating for irregularities or unevenness in thickness of the workpiece so as to exert a uniform pressure across the panel. Self-compensating cylinders have been devised for use with a clamp to balance the pressure or force applied across the clamping area; and representative patents showing different types of clamping devices are shown in U.S. Pat. No. 3,338,573 to Lukas, U.S. Pat. No. 2,209,379 to M. A. Bell and U.S. Pat. No. 3,056,600 to J. I. Merrick. None, though, suggests a way of employing the device as a part of a work holder for large items. In this relation, in clamping large workpieces it is desirable to position the clamping device at a point spaced as far away from the supported end of the workpiece as possible and to afford the widest possible range in movement of the clamping device in moving into engagement with the workpiece. For instance, in clamping a door panel, it is most effective to support the bottom edge of the door while exerting a clamping force across a broad surface adjacent to the upper or opposite end of the door.